If you have a shottie that is chambered to take 3 1/2" shells you measure the capacity using 3 1/2
' shells. Therefore you can get more in if you load with 2 1/2" shells. Not sure where you stand legally though if you put more than 7 2 1/2" shells into a 3 1/2" mag. I imagine at that point you break the law. A bit like the old rules where the high capacity mag was legal until you put it into an A cat rifle.
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